The present invention relates to a drum assembly for an electrostatic copier. More particularly this invention concerns an arrangement for releasably mounting a light-sensitive copy drum in an electrostatic copying apparatus.
An electrostatic or xerographic copier such as described in commonly assigned U.S. Pat. No. 3,734,604 has a cylindrical drum provided on its outer surface with a light-sensitive coating. It is essential that the surface carrying this coating be perfectly cylindrical and rotate about a perfectly stationary axis. Thus recourse has earlier been had to massive drums which are painstakingly machined and supported on heavy-duty bearings.
In recent times in an effort to save costs it has been suggested to provide the light-sensitive coating on a relatively light-duty tubular drum that itself is supported on a support sleeve. Thus if the surface of the drum is damaged it is possible to exchange it with another tubular drum that costs a great deal less than the whole drum assembly. In use of an electrostatic copier an occassional paper jam frequently will damage the thin drum coating and necessitate changing the drum and vacuum-plating a new coating of selenium on the damaged drum.
The problem with these known arrangements is that the drum must be meticulously and painstakingly secured in place on the support sleeve. Furthermore this support sleeve must in turn be adequately supported in the stationary copier frame, so that the complex structure which makes exchanging the drum possible is in itself quite expensive. The sleeve must be very rigid and can only be rotatably supported at one of its ends, as the drum must be able to slip over the other free end.
One solution to this difficulty has been to provide guide rollers which ride on that surface of the drum turned toward the lens system that casts the image to be copied on the drum. These guide rollers insure that the critical portion of the drum is properly positioned. This arrangement has proven itself unsatisfactory in practice and often leads to damaging of the delicate selenium coating on the drum.